One interesting plotline could be the sister having special powers that she has to hide from the Cullens. Maybe she can communicate with sea creatures and this causes some confusion and mystery in the story.
A possible plotline could also be that the sister discovers she is a siren at a crucial moment, like when the town is in danger. She then has to learn to control her powers quickly in order to save everyone, while also dealing with the fact that her newfound identity might change how her family and friends see her.
Another plotline might involve the sister being in a love triangle with a werewolf and a vampire, just like Bella was, but with her own unique twist because of her siren nature. For example, her siren powers might attract both of them in different, more intense ways.
One possible plotline could be the exploration of the relationship between Bella and her vampire sister. Maybe they have to face some challenges together, like dealing with other vampire clans who see them as a threat. Another could be their journey in discovering their unique powers as sisters in the vampire world.
Well, obviously Bella Swan is one of the main characters. Her sister, who is a siren, is also a main character. Then there would likely be characters from the Twilight series like Edward Cullen, Jacob Black, etc. as they interact with Bella and her sister.
A common theme might be the reaction of the other Twilight characters to Bella as a siren. Edward, for instance, could be both fascinated and worried about this new side of her. Also, the Volturi might see her as a threat due to her siren abilities. This could lead to conflicts and power struggles within the Twilight universe.
In some Bella Swan pregnant fanfiction, the plot could revolve around Bella's human friends' reactions. They might be shocked and scared at first, but then gradually come to accept the situation. And there could be a lot of drama about how Bella is going to balance her human life with this new, very different situation of being pregnant with a half - vampire baby. There could also be a focus on her relationship with Edward during this time, like how they grow closer or face new challenges in their relationship because of the pregnancy.
One interesting plotline could be Bella being reborn with some sort of supernatural power. For example, she might be able to control the elements. This would completely change the dynamic of her relationships in Forks. Another could be that she remembers her past life and uses that knowledge to avoid some of the dangerous situations she got into in the original story, like with the Volturi.
In some 'Bella Swan as a Vampire' fanfiction, a great plotline is her exploration of her new vampire senses. She could find that her sense of smell leads her to discover things about the past of the places she visits. And in terms of relationships, her bond with Edward could change in a really interesting way. Since they are both vampires now, they might have different power dynamics and new challenges in their relationship. For instance, they could compete in terms of their vampire skills, which could either strengthen or strain their relationship.
A possible plotline could be that the Winchester sisters are magically transformed into Bella Swan right before a major event in the 'Twilight' series. They then have to quickly adapt to her life and relationships while also trying to figure out a way to get back to their own world. They might use their combat skills to protect themselves from the vampires and werewolves in a more aggressive way than Bella usually does.
Edward can be really interesting. His relationship with Bella takes on a new dimension when she's a siren. He has to grapple with his love for her while also being cautious of her new powers. Another character could be Jacob. His loyalty to Bella might be tested as she becomes something different, and he might have his own mixed feelings about her siren side.
Well, one possible plotline could be Bella Swan's struggle to adapt to the strange ways of the Creeper Mates. Maybe she has to learn their customs and language. Another could be a love triangle between Bella and two of the Creeper Mates. Or perhaps there's a mystery within the Creeper Mates' society that Bella helps to solve.
There could also be a plotline where Bella tries to understand why she was made the unwanted bride in the first place. She delves into the history and politics of the vampire or werewolf world (depending on the context of the fanfiction) and uncovers secrets that not only help her deal with her current situation but also change the entire power dynamic in that world.